.rectangle {
    width: 370px;
    margin: 50px auto;
    font-size: 1.1em;
  }
  
  input, label, span {
    margin: 5px 0;
    font-size: 1em;
    padding-left: 5px;
  }
  
  #rectangle-calc {
    width: 70px;
    margin-left: 63px;
  }
  
  span {
    color: red;
  }
  
  #rectangle-width-validate,
  #rectangle-height-validate {
    color: red;
    margin-left: 63px;
    font-size: 0.8em;
  }